The present invention relates to a new method for DNA purification. The method according to the invention enables pharmacologically usable double-stranded DNA to be purified rapidly. More especially, the purification method according to the invention involves a specific hybridization between a sequence of the DNA and an oligonucleotide.
Gene and cell therapy techniques are currently undergoing remarkable development. However, these techniques entail the possibility of producing large amounts of DNA of pharmaceutical purity. In effect, in these new therapies, the medicament often consists of DNA itself, and it is essential to be able to manufacture it in suitable amounts, to isolate it and to purify it in a manner suited to therapeutic use in man.
In recent years, the feasibility of injection of plasmid DNA for gene therapy or vaccination has been demonstrated by numerous reports demonstrating that DNA expression vectors can be taken up by various cell types and genes encoded by these plasmids can be subsequently expressed (Ledley, 1995 Hum. Gene Ther. 6, 1129).
The genes of interest for gene therapy or vaccination applications may include, for example, tumor suppressor gene, suicide genes, or anti-sense sequences. They can also encode proteins such as alpha-fetoprotein AFP (Morinaga, 1983, Proc. Natl. Acad. Sci. USA, 80, 4604), enzymes, hormones, cytokines, growth factors such as FGF (Jouanneau et al., 1991, Proc. Natl. Acad. Sci. USA, 88, 2893) or VEGFB (Olofsson B al., 1996, Proceedings 93, 576), clotting factors such as B-deleted Factor VIII (Truett et al., 1985, DNA 4, 333), apolipoproteins, neurotransmitters, neurotrophic factors, natural or chimeric immunoglobulin. Reporter genes such as lacZ encoding the Escherichia coli β-galactosidase are also used.
Major challenges for using plasmid DNA as a gene delivery vector in human are i) the manufacture and ii) the purity of this drug product. Technologies for the production of plasmids vectors with high copy number in Escherichia coli hosts have been recently developed. The plasmids currently used are either ColE1-derived plasmids such as pBR322, pUC or pBluescript (Lahijani et al., 1996, Hum. Gene Ther., 7, 1971) or pCOR plasmids (Soubrier et al., 1999, Gene Therapy, 6, 1482).
The second concern raised by the use of plasmid DNA as a gene therapy vector is the purity of the plasmid vector itself. Current purification methods such as ultracentrifugation in CsCl gradients or chromatography can be inefficient in removing contaminants such as host genomic DNA and RNA or proteins. Particularly, host genomic DNA whose chemical structure is very close to that of plasmid DNA, is extremely difficult to remove using classical chromatography. Typical concentrations of up to 0.5 to 1% host genomic DNA are found in plasmid preparations obtained by classical chromatography. Therefore, in order to develop plasmid DNA as a safe vector for human gene therapy, there is a need for purification technologies that will lower the content of host genomic DNA down to much lower levels, typically 0.1% or even 0.01% or lower.
The present invention describes a simple and especially effective new method for DNA purification. It makes it possible, in particular, to obtain especially high purities with high yields. The method according to the invention is based essentially on a specific interaction between a sequence inserted into the DNA to be purified and an oligonucleotide composed of natural or modified bases.
It has recently been shown that some oligonucleotides are capable of interacting specifically in the wide groove of the DNA double helix to form triple helices locally, leading to an inhibition of the transcription of target genes (Hélène et Toulmé, Biochim. Biophys. Acta 1049 (1990) 99). These oligonucleotides selectively recognize the DNA double helix at oligopurine-oligopyrimidine sequences, that is to say at regions possessing an oligopurine sequence on one strand and an oligopyrimidine sequence on the complementary strand, and form a triple helix locally thereat. The bases of the third strand (the oligonucleotide) form hydrogen bonds (Hoogsteen or reverse Hoogsteen bonds) with the purines of the Watson-Crick base pairs.
A use of this type of interaction to isolate a plasmid has been described in the prior art. Thus, Ito et al. (PNAS 89 (1992) 495) describe the use of biotinylated oligonucleotides capable of recognizing a particular sequence of a plasmid and of forming a triple helix therewith. The complexes thus formed are then brought into contact with streptavidin-coated magnetic beads. Interaction between the biotin and the streptavidin then enables the plasmid to be isolated by magnetic separation of the beads followed by elution. However, this method has some drawbacks. In particular, two successive specific interactions are needed, the first between the oligonucleotide and the plasmid and the second between the biotinylated complex and the streptavidin beads. Furthermore, the final solution may be contaminated with biotinylated oligonucleotide, which cannot be used in a pharmaceutical composition.